Additives to negative photoresists which increase the sensitivity thereof

ABSTRACT

The use of a scanning electron beam to generate a pattern in a negative photoresist is known. Electron beam equipment can be made which is capable of scanning very quickly, but standard negative photoresists require such a large flux of electrons for proper exposure that the scanning equipment must be operated at speeds substantially slower than the capability of the equipment. By adding certain compounds which dissociate readily into free radicals to the photoresist, the sensitivity or speed of the photoresist is effectively increased. As a result, the electron beam can scan at a higher rate. Compounds which are most effective are benzophenone, benzil and 1,4-diphenyl-1,3butadiene.

United States Patent [191 Broyde ADDITIVES TO NEGATIVE PHOTORESISTSWHICH INCREASE THE SENSITIVITY THEREOF Inventor: Barret Broyde, LawrenceTownship,

Mercer County, NJ.

Western Electric Company, Incorporated, New York, NY.

Filed: Feb. 4, 1974 Appl. No.: 439,660

Assignee:

US. Cl 96/115 R, 96/35.l, 96/362,

96/91 N, 117/9331, 204/159.18, 252/500 Int. Cl G03c 1/68, G03c 1/70Field of Search 96/115 R, 91 N; 252/500 References Cited UNITED STATESPATENTS 2/1954 Minsk et a1. 96/115 R 2/1954 Minsk et a1. 96/1 15 R9/1958 Hepher et a1. 96/91 N 9/1970 Sloan 96/115 R [451 Feb. 11,1975

3,594,243 7/1971 Deutsch et a1 96/35.1

OTHER PUBLICATIONS Primary Examiner-Ronald H. Smith Attorney, Agent, orFirm-J. Rosenstock [57] ABSTRACT The use of a scanning electron beam togenerate a pattern in a negative photoresist is known. Electron beamequipment can be made which is capable of scanning very quickly, butstandard negative photoresists require such a large flux of electronsfor proper exposure that the scanning equipment must be operated atspeeds substantially slower than the capability of the equipment. Byadding certain compounds which dissociate readily into free radicals tothe photoresist, the sensitivity or speed of the photoresist iseffectively increased. As a result, the electron beam can scan at ahigher rate. Compounds which are most effective are benzophenone, benziland 1,4-diphenyl-1,3-butadiene.

6 Claims, 3 Drawing Figure RESIST THICKNESS ()1 PARTIALLY CYCLIZED CISPOLYISOPRENE a 1.0% BENZOPHENONE 0.4 I

I PARTIALLY CYCLIZED CIS POLYISOPRENE 0.1 I l l A I l I I l l 0 2 4 s 8IO I2 I4 I6 DOSE DENSITY (,U COULOMBS /CM RESIST THICKNESS (M) PARTIALLYCYCLIZED CIS POLYISOPRENE BI I.O BENZIL PARTIALLY CYCLIZED CISPOLYISOPRENE RESIST THICKNESS (MI PARTIALLY CYCLIZED CIS POLYISOPRENE (a01% |,4- DIPHENYLIJ-BUTADIENE 0.4 z

0.2 PARTIALLY CYCLIZED CIS POLYISOPRENE 1 0| I I I I I I 02468|OI2|4I6DOSE DENSITY I C0ULOMBS/CM2) ADDITIVES TO NEGATIVE PHOTORESISTS WHICHINCREASE THE SENSITIVITY THEREOF CROSS REFERENCE TO RELATED APPLICATIONThis is a division of application Ser. No. 344,790 filed Mar. 26, 1973,now US. Pat. No. 3,808,155, which is a continuation-in-part of mycopending application, Ser. No. 137,032, filed Apr. 23, l97l, saidapplication being a continuation of my application Ser. No. 764,866,filed Oct. 3, 1968, both now abandoned. Said copending application isassigned to the same assigned as the instant application.

BACKGROUND OF THE INVENTION Thornley 1. Field of the Invention Thisinvention relates generally to additives to negative photoresists whichincrease the sensitivity thereof and, more particularly, the inventionrelates to additives to standard negative photoresists which result inincreased reactivity of the photoresist. The invention has particularapplication in, but is not limited to, the generation of microminiaturecircuit patterns by electron beam exposure of negative photoresists.

A negative photoresist is an organic material which, when exposed toradiation, undergoes chemical reactions of the type referred to ascrosslinking, which reactions result in insolubilizing the exposedphotoresist. The crosslinking reactions are of the type that can beinitiated either by light or by electrons. Because it is possible togenerate electron beams of substantial energy but only 0.1; or smallerdiameter, their use in the generation of extremely small circuitpatterns is preferred to the use of light. Electron beams also have amuch better resolution capability than is possible when using an opticalmask and light exposure, and they have a much greater depth of focus.The exposure of a con ventional positive photoresist involvessolubilization of the exposed areas, and the chemical reactions involvedare of the scission or degradation type, which also require absorptionof light or electrons. Because this type of photoresist requires higherflux densities for proper exposure than negative photoresists require,electron beams are not widely employed in this service. Materials thathave been successfully used as electronsensitive positive photoresistsare discussed by Haller et al., IBM Journal, May 1968, pp. 251-256.

The most common negative photoresist in current use are KodakPhotoresist (KPR, KPRZ, KPR3, trademarked products of Eastman KodakCompany) and Kodak Thin Film Resist (KTFR, trademark product of EastmanKodak Company). The KPR composition is based on the dimerization ofpolyvinyl cinnamate, while KTFR is based on the crosslinking of apolymerized isoprene dimer, i.e., partially cyclized cispolyisoprene,averaging one double bond per car bon atoms. Another member of the KPRgroup besides KPR 2 and 3, is KOR (trademark for Kodak Ortho Rcsist).Another product, KMER (trademark for Kodak Metal Etch Resist) belongs tothe KTFR group. The invention will be described with primary referenceto use of polyvinyl cinnamate and partially cyclized cispolyisoprene,but it will be appreciated that it is not so limited.

The crosslinking and insolubilization of resists is a I complexphenomenon, but is believed to be describable, broadly, as follows. Apolyvinyl cinnamate or KPR-type resist has the following generalformula:

H e t).

The number average molecular weight (N.A.M.W.) is 180,000-230,000, andthe weight average molecular weight (W.A.M.W.) is 3l5,000-350,000. Uponexposure to light or electron energy, a diradical is formed:

where A is the vinyl cinnamate monomer (structure 1 where n l). Thediradical then reacts with another diradical to form a 4-member ring:

These materials (averaging one double bond per carbon atoms) have aN.A.M.W. of 65,000 $5,000 and a W.A.M.W. of about 120,000, and areinsolubilized by free radical reactions. Thus, radiation produces adiradical:

where B is the monomer of (4). The diradical reacts with other moleculesuntil the free radical terminates. For good resolution, additives may beincorporated to keep the chain short. In all of the above structuralformulae, the subscripts (n, m, p, s, I) refer to integers which aredeterminative of molecular weight. While polyvinyl cinnamate andpartially cyclized cispolyiosprene are insolubilized by differentmechanisms, both result in crosslinked systems.

The procedures for generating a microminiature pattern circuit byelectron bombardment of a photoresist are well established, and aresummarized briefly below. The substrate is typically an oxidized siliconwafer or a chromium-coated glass plate. The photoresist is dissolved ina suitable solvent and applied to the substrate, which may then be spunat a high speed to leave an even film of the photoresist, having acontrolled thickness, on the substrate surface. Alternatively, thephotoresist-solvent solution may be sprayed on. In either case, most ofthe solvent evaporates immediately. The photoresist-coated substrate isthen dried or baked briefly to drive off any remaining solvent and toimprove adhesion. The coated substrate is then placed in a vacuumchamber and, when the vacuum has been established, it is radiated in thedesired pattern and with an appropriate dosage. The coated and radiatedsubstrate is then placed in a developer, which is a solvent for thesoluble portion of the resist, to dissolve and remove the unexposedportions. It is again dried or baked. The desired pattern area on thesubstrate is now free of any covering film, and etching, plating oroxidizing follows. After this step, the remaining resist is strippedoff.

There are a variety of limitations imposed upon the radiation step, butthese are fullycovered in the prior art (listed below) and need only besummarized here.

Briefly, the amount of radiation must fully expose the photoresist allthe way down to the substrate, or else the developed photoresist willfloat off when the underlying, undeveloped photoresist is dissolved inthe developer. On the other hand, too much radiation will causestripping problems and even polymer degradation. The amount of radiationnecessary to form an insoluble photoresist is a function of themolecular weight of the material, and the gross amount of radiation. Theefficiency of the crosslinking reactions is related to the acceleratingpotential of the electrons, penetration range (also a function ofpotential) and other factors. For instance, it has been determined thatthe maximum film thickness that can be developed by 5 KV electrons isabout 6,500A, and by 10 KV electrons is about 21L. On the other hand,photoresists should initially be at least 6,000A thick to avoid pinholeproblems (a 6,000A film will shrink to about 4,000A when developed).Other limitations which must be considered are electron scatter withinthe film and back-scatter fromthe substrate,

though these are of a lesser order.

,2. Discussion of the Prior Art Prior workers have carried out extensivestudies on the foregoing limitations, particularly with respect to thesensitivity and resolution capability of standard resists. This workneed not be described herein, but is referenced below for backgroundinformation:

None of these prior workers have made any effort to alter conventionalphotoresist compositions, although it is significant to note thatThornley et a1. appreciated the problems which they pose: For serialexposures, such as may be required in printed circuit generators, themaximum exposure rates are limited by the sensitivities of presentlyavailable resists. (Thornley et al., op

cit,p. 1151).

While prior workers who have studied electron beam development ofresists to generate small patterns have worked only with the availableresists, workers in the field of photolithography, where photoresistswere first employed, have proposed literally thousands of compounds asphotopolymerization initiators, catalyzers and sensitizers. The end inview was generally to in crease the sensitivity or resolution of thephotoresist to light of a particular wavelength. This work is notreadily summarized, but the following US. Pat. Nos. are consideredrepresentative: 2,816,091; 2,831,768; 2,861,057; 3,168,404; 3,178,283;3,257,664; and 3,331,761.

OBJECTS OF THE INVENTION Ageneral object of the present invention is toprovide new and improved additives tonegative photoresists whichincrease the sensitivity thereof to electrons.

A further object of the present invention is to provide additives tostandard negative photoresists which result in increased reactivity ofthe photoresist itself.

Another object of the present invention is to improve the sensitivity ofa standard negative photoresist by inv eluding novel additives therein.

A further object of the present invention is to reduce the flux densityand, hence, the exposure time required to fully expose a standardphotoresist, by incorporating novel additives therein.

Various other objects and advantages of the invention will become clearfrom the following detailed description of several embodiments thereof,and the novel features of the invention will be particularly pointed outin connection with the appended claims.

THE DRAWINGS FIGS. l-3 are plots of resist thickness vs. flux densityfor exposure of 6,000A films of partially cyclized cispolyisoprene andpartially cyclized cis-polyisoprene plus the preferred additives of theinvention.

SUMMARY AND DESCRIPTION OF EMBODIMENTS 1,4-(11 heny1-1,3- 1t 11 irf f citli buta lane The amount of the additive used is important. If toolittle additive is present, sufficient free radicals will not begenerated to cause a maximum effect. On the other hand, if too much ofthe additive is present, the free radicals will react with each otherrather than with the resist, and crosslinking will not be aided. It hasbeen determined that no more than about 5% (all percentages are weightpercent) of the additive should be'added to either the polyvinylcinnamate resist-solvent mixture, or the partially cyclizedcis-polyisoprene resist-solvent mixture. It should be understood,however, that this may amount to oreven 50% of the respective resistafter the solvent is removed. Generally, a 1% solution of the additiveis preferred.

If one knows the average molecular weight of the photoresist film andthe electron accelerating potential, and makes certain assumptionsregarding electron penetration, scatter and energy transfer, the geldose of energy can be calculated from theory (the gel dose is theelectron flux necessary to record an image in the film surface, i.e.,the minimum dose to cause insolubility). Experimental results are infair agreement with such calculations. When an additive causes a largenumber of free radicals to be formed at each collision of an electronwith a molecule, then it is not unreasonable to expect that the numberof molecules crosslinked at each such energy transfer site will behigher. The problem, as noted above, is to add compounds that will notreact spontaneously. While increased crosslinking could be expected withproper additive selection, the magnitude of improvement achieved withthe abovenoted compounds is quite remarkable. In particular, a five-foldreduction in the dose density required to fully expose a 4,000A film isachieved. The improvement is not linear; the gel dose is reduced littleif at all by using the additives. These facts are all clear in thefollowing specific examples.

A further requirement of the additive is that it be soluble in thesolvent system employed with the particular resist. The three notedcompounds satisfy this requirement.

Both polyvinyl cinnamate and partially cyclized cispolyisoprene aredissolved in a solvent thereof. With the latter, a thinner may also beemployed; this acts merely to reduce viscosity and produce a thinnerfilm. The solvent system used for polyvinyl cinnamate is 86-87%chlorobenzene and 13-14% cyclohexanone. The partially cyclizedcis-polyisoprene solvent system is 12% ethylbenzene, 82% mixed xylenesand 6% methylcellosolve. Both systems also contain a sensitizer; inpartially cyclized cis-polyisoprene (commercially available as KTFR)this is believed to be 2,6 bis (pazidobenzilidene)4-methylcyclohexanone.The partially cyclized cis-polyisoprene thinner is primarily mixedxylenes.

EXAMPLE I To establish a basis for comparison, tests were first madewith polyvinyl cinnamate photoresists without any additives. A polyvinylcinnamate resist-solvent so lution was applied to a chromium-coatedglass plate. The resist-solvent solution was commercially obtained andcomprised polyvinyl cinnamate (N.A.M.W. of 180,000 to 230,000; W.A.M.W.of 315,000 to 350,000) dissolved in 86-87% chlorobenzene, and 13-14%cyclohexanone. The coated glass plate was then spun so that theresulting coating, after baking at C for 10 minutes, was 6,000A thick.The coated plate was then placed in a vacuum chamber and radiated withelectrons accelerated at 15 KV. The plate was developed with a polyvinylcinnamate developer, commercially obtained, and baked at 150C for 10minutes. The following results were obtained:

a. Flux needed to record an image (gel dose) 1.1

X 10' coul/cm. b. Flux needed to form 3,000A thick resist layer 6 X 10coul/cm c. Flux needed to form maximum thickness (after development,4,000A) resist 10 X 10' coul/cm.

EXAMPLE II To establish a basis for comparison, tests were first madewith partially cyclized cis-polyisoprene photoresist without anyadditives. A partially cyclized cispolyisoprene photoresist-solventsolution was mixed with a thinner (mixed xylenes) in a l to 3 ratio. Theresist-solvent solution was commercially obtained and comprisedpartially cyclized cis-po1yisoprene (averaging one double bond per 10carbon atoms; N.A.M.W. of 65,000 i 5,000; W.A.M.W. of about 120,000)dissolved in 12% ethylbenzene, 82% mixed xylenes and 6% methycellosolve.The mixture was applied to a chromium-coated glass plate (or,alternatively, to a silicon slice onto which a 18,000A SiO layer hadbeen grown), and then spun to a thickness of 8,000A. After baking at150C for 10 minutes, the film was 6,000A thick. The coated plates werethen put into a vacuum chamber and radiated with 15 KV electrons. Theplate was developed with a partially cyclized cispolyisoprene developer,commercially obtained, and a partially cyclized cis-polyisoprene rinse,commercially obtained, and baked at 150C for l minutes. The followingresults were found:

a. Flux needed to record image (gel dose) 0.9 X

l0 coullcm b. Flux needed to form 3,000A film coul/cm c. Flux needed toform maximum (4,000A) thickness 7.5 X 10 coul/cm Under identicalconditions, but with 5 KV electrons, the dose densities required toexpose partially cyclized cis-polyisoprene films were:

a. 0.5 X coul/cm b. 0.75 X 10' coul/cm c. 2 X 10" coul/cm EXAMPLE IIIThe procedure of Example I was repeated except that a 5 weight percentsolution of benzophenone in the polyvinyl cinnamate resist-solventsolution was prepared and employed. The three tests noted in Example Iwere carried out (with KV electrons). The results were as follows:

a. 1.0 X l0 coul/cm b. 1.5 X 10' coul/cm c. 2.0 X 10' coul/cm' EXAMPLEIV The procedure of Example 11 was repeated except that a 1 weightpercent solution of benzophenone in the thinned (1 to 3) partiallycyclized cis-polyisoprene photoresist-solvent solution was prepared andemployed. Dose densities for the three tests with 15 KV electrons wereas follows:

a. 0.5 X 10 coul/cm b. 1.0 X 10 coul/cm c. 1.75 X 10 coul/cm Theimprovementachieved by this additive is graphically illustrated in FIG.1.

If 5 KV electrons are used instead of 15 KV electrons, results for thethree tests are as follows:

a.'0.45 X 10' coul/cm b. 0.5 X 10" coul/cm c. 0.75 X 10 coullcm Reasonsfor the higher efficiency of lower-energy electrons, and reasons forpreferring 15 KV beams, are discussed below.

EXAMPLE V The procedure of Example 11 was repeated except that a 1weight percent solution of benzil in the thinned partially cyclizedcis-polyisoprene resist-solvent solution was preparedand employed.Results of the three tests are as follows:

a. 0.5 X 10 coul/cm b. 1.0 X 10 coul/cm c. 1.5 X 10 coullcm Theimprovement achieved with this additive is graphically illustrated inFIG. 2.

EXAMPLE VI The procedure of Example II was repeated except that a 0.1weight percent solution of 1,4-diphenyl-1 ,3 butadiene in the thinnedpartially cyclized cispolyisoprene photoresist-solvent solution wasprepared and employed. Dose densities for the three tests were asfollows:

a. 0.5 X 10 coul/cm b. 1.5 X 10* coul/cm c. 1.5 X 10' coul/cm Theimprovement achieved with this additive is illustrated in FIG. 3.

The magnitude of improvement brought about by each of the additives isreadily seen in Table I, where the percent reduction in dose for each ofthe three levels, as compared to the photoresist without any additives,is setforth.

It will be noted that one of the effects of the additives of the presentinvention is to increase the slope of the plot of resist thickness vs.dose density to near infinity near the gel point (see FIGS. l-3). Byusing the minimum dose density needed to achieve the desired thick ness,back-scattered electrons or scattered primary electrons are minimized ifnot eliminated, and resolution capability of the resist viscorrespondingly increased. Under these conditions, an edge definition ofabout 300A can be expected as an upper limit. This is significantlybetter than previously reported definition.

It will be further noted by comparing the partially cyclizedcis-polyisoprene radiated with 5 and 15 KV electrons, that the 5 KVsamples required less energy at all three stages. It is quite true, infact, that lower energy electrons act much more efficiently than higherenergy electrons; on the average,about 2.5 times the number of moleculesat each energy transfer point will react at 5 KV than will at 15 KV. Itwould seem appropriate, then, to utilize lower energyelectrons, butcontrol of the size of the beam is more difficult at low energies. Ifvery high potentials are used (+20 KV) the efficiency of crosslinkingdrops too low and back-scatter can become a significant problem. Forthese reasons, a 15 KV accelerating potential is preferred.

It is to be understood that various changes in the details, steps,materials and arrangements of parts, which have been herein describedand illustrated in order to explain the nature of the invention, may bemade by those skilled in the art within the principle and scope of theinvention as defined in the appended claims and their equivalents.

What is claimed is:

1. An electron sensitive photoresist composition comprising a partiallycyclized cis-polyisoprene resist and a compound selected from the groupconsisting of benzil and benzophenone, said resist and said compoundbeing dissolved in a solvent system employed with said resist, theconcentration of said compound in 4. The method as defined in claim 3wherein said resist and said compound are dissolved in a solvent systememployed with said resist.

5. The method as defined in claim 4 wherein said compound is present insaid photoresist-solvent system in an amount ranging from 0.1 to 5%.

6. The method as defined in claim 4 wherein said solvent systemcomprises 12% ethylbenzene, 82% mixed xylenes and 6% methylcellosolve.

1. AN ELECTRON SENSITIVE PHOTORESIST COMPOSITION COMPRISING A PARTIALLYCYCLIZED CIS-POLYISOPRENE RESIST AND A COMPOUND SELECTED FROM THE GROUPCONSISTING OF BENZIL AND BENZOPHENONE, SAID RESIST AND SAID COMPOUNDBEING DISSOLVED IN A SOLVENT SYSTEM EMPLOYED WTIH SAID RESIST, THECONCENTRATION OF SAID COMPOUND IN THE PHOTORESIST-SOLVENT SYSTEM BEINGAN AMOUNT RANGING FROM 0.1 TO 5%.
 2. The composition as claimed in claim1 wherein said resist and said compound are dissolved in a solventsystem comprising 12% ethylbenzene, 82% mixed xylenes and 6%methylcellosolve.
 3. A method of increasing the electron sensitivity ofa photoresist comprising a partially cyclized cis-polyisoprene resistwhich comprises combining the photoresist with a sensitizing compoundselected from the group consisting of benzil and benzophenone.
 4. Themethod as defined in claim 3 wherein saidd resist and said compound aredissolved in a solvent system employed with said resist.
 5. The methodas defined in claim 4 wherein said compound is present in saidphotoresist-solvent system in an amount ranging from 0.1 to 5%.
 6. Themethod as defined in claim 4 wherein said solvent system comprises 12%ethylbenzene, 82% mixed xylenes and 6% methylcellosolve.